1. Field of the Invention
The present invention relates to a mobile object position detecting system such as a vehicle position detecting system or a vehicle identification system
2. Description of the Related Art
Recently, various systems have been developed as intelligence transport system (ITSs). One of such systems is an electronic toll collection (ETC) system.
In the tollgates of an ETC system, determination of whether a vehicle is an ETC vehicle or a non-ETC vehicle is important. For realizing this, an ETC system is suggested (see: A. Mitsuno et al., "Direction Finding Technology and Application to Radio-communication Vehicle Identification System", NEC Technical Report, Vol, 50, No. 7, pp. 147-155, 1997). That is, a vehicle detector is provided for optically detecting whether or not a vehicle is present in a communication zone. In this case, the vehicle has an ETC transceiver. Also, a camera is provided to take a photograph of a vehicle. The transceiver is mounted on a dashboard of the vehicle.
Also, first and second antennas are installed in a row perpendicular to a lane on a pole. A signal is transmitted from the transceiver via the first antenna to a modulator/demodulator or vice versa. That is, the signal received at the first antenna from the transceiver is amplified, demodulated, and transmitted to a personal computer. On the other hand, a signal from the personal computer are modulated and transmitted from the first antenna to the transceiver.
A signal is supplied from the transceiver via the second antenna to a direction of angle (DOA) detector for calculating an angle of the vehicle. Each of the angles detected by the DOA detector is transmitted to the personal computer.
Further, a signal from the vehicle detectors is supplied to the personal computer, and the personal computer transmits a control signal to the camera.
The prior art ETC system will be explained later in detail.
In the prior art ETC system, however, the DOA detector is constructed on the basis of the principle of an interferometer (see FIG. 2 of the above-mentioned document). As a result, the DOA detector is large in size and high in cost. This also makes the ETC system large in size and high in cost.